Urine Output Calculator (mL/kg/hr)
Urine Output Calculator (mL/kg/hr)
Calculate urine output in mL/kg/hr and apply the KDIGO thresholds used to define acute kidney injury.
Urine Output (mL/kg/hr)
Volume ÷ (weight × time)Urine volume 320 mL, body weight 70 kg, over 8 hours
Formula
- volume
- mL of urine collected over the period
- weight
- kg — use a dry or usual weight where available, not a fluid-overloaded current weight
- hours
- the collection period, e.g. hourly, 6-hourly or over a shift
Worked example
Urine volume 320 mL, body weight 70 kg, over 8 hours
70 × 8 = 560
320 ÷ 560 = 0.57 mL/kg/hr
0.5–2 mL/kg/hr → normal
KDIGO urine output criteria for AKI
| Urine output | Duration | KDIGO stage |
|---|---|---|
| < 0.5 mL/kg/hr | 6–12 hours | Stage 1 |
| < 0.5 mL/kg/hr | ≥ 12 hours | Stage 2 |
| < 0.3 mL/kg/hr, or anuria | ≥ 24 hours, or ≥ 12 hours anuria | Stage 3 |
An earlier signal that is easy to lose
The KDIGO definition of acute kidney injury has two arms — a rise in serum creatinine and a fall in urine output — and the urine output criterion is the more time-sensitive of the two. Creatinine takes hours to days to reflect a fall in filtration, since it depends on production, distribution and the time needed to reach a new steady state; urine output changes far sooner, which makes it the earlier warning sign when it is measured accurately.
That accuracy is the practical problem. The criterion depends on precise, hourly-charted volumes against a reliable weight, which is routine in critical care with a urinary catheter but is often not recorded with the same rigour on a general ward, where output may be estimated, intermittently charted, or based on a weight that has drifted with fluid overload. A falsely reassuring or falsely alarming reading is common outside monitored settings, so a single low value should prompt closer observation and a check of the weight and volume used rather than an immediate escalation.
At the high end, urine output above roughly 2 mL/kg/hr is not automatically reassuring either. Diuretic use, osmotic diuresis from hyperglycaemia, diabetes insipidus, post-obstructive diuresis after relief of a blocked urinary tract, and the polyuric recovery phase of acute tubular necrosis can all drive high output, and several of these carry their own electrolyte and volume risks that need separate management.
Frequently asked questions
What urine output defines AKI?
By KDIGO criteria, under 0.5 mL/kg/hr for 6–12 hours is stage 1, under 0.5 mL/kg/hr for 12 hours or more is stage 2, and under 0.3 mL/kg/hr for 24 hours (or anuria for 12 hours) is stage 3.
Is urine output or creatinine a better AKI marker?
Neither alone — they are two arms of the same definition. Urine output changes sooner than creatinine, but is more easily measured inaccurately outside critical care, so both should be tracked together.
What weight should be used for the calculation?
A dry or usual weight where it is known. A current weight inflated by fluid overload will understate the true mL/kg/hr output and can mask developing oliguria.
What causes high urine output?
Diuretics, osmotic diuresis including hyperglycaemia, diabetes insipidus, post-obstructive diuresis, and the recovery phase of acute tubular necrosis are the common causes, several of which carry their own fluid and electrolyte risks.
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References
- Kidney Disease: Improving Global Outcomes (KDIGO) Acute Kidney Injury Work Group. KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney Int Suppl. 2012;2(1):1–138.
- Md Ralib A et al. The urine output definition of acute kidney injury is too liberal. Crit Care. 2013;17(3):R112.